A representative prior art fuel pump of the crescent gear type is illustrated diagrametically and described in U.S. Pat. No. 3,566,901. In this type of lift pump, fuel is drawn from a supply, such as a tank mounted relatively lower than the pump, through an inlet port into the pump's housing by means of relatively rotatable pumping elements. The latter are mounted within a retaining ring and include inner and outer gears. The inner gear is fixed on a drive shaft whose axis is disposed eccentrically relative to the outer gear so that to one side of the shaft the teeth of the outer gear and inner gear mesh. To the opposite side of the shaft, is a space containing a crescent-shaped member allowing fuel to be transported in fluid chambers defined between the teeth on both the outer gear and inner gear. With rotation of the drive shaft, fuel is carried within these fluid chambers from the pump inlet to a discharge chamber with the meshing teeth of the gears pressurizing fuel in the discharge chamber. The latter communicates with a pressure regulator disposed within the housing and the regulator includes a spool movable axially within a bore. Under the urging of a spring reacting against one end of the bore, the spool is urged toward a position against a valve seat at the other end of the bore. Upon start up of the pump, when the pressure of the fuel discharged from the pump elements is sufficient to overcome the spring, the spool is shifted away from the valve seat thereby allowing high pressure fuel to be delivered to a burner nozzle communicating with a nozzle port. As the spool is shifted against the spring, a return port in the housing may be opened to allow excess fuel to be either returned to the inlet port, as in a one pipe lift fuel supply system, or to the supply tank as in a two pipe lift system. For fuel pressure adjustment purposes, an adjusting screw at the one end of the bore engages a movable spring seat so that the force of the spring acting against the valve spool may be adjusted to increase or decrease fuel pressure as desired to meet the pressure requirements of the particular heating system within which the pump is to be used.
Typically, pumps of the foregoing type are utilized in heating systems requiring fuel to be delivered at high pressure (100 psi-200 psi) but have a minimum firing rate of approximately 0.5 gph (gallons per hour) in order to avoid clogging of burner nozzle passages. Heretofore, it is by restricting the size of the burner nozzle passages that the minimum firing rates of 0.5 gph have been achieved with a crescent-type gear pump without the use of some form of auxiliary metering device. For example, U.S. Pat. No. 2,766,693 discloses a fuel pumping system which, in addition to a crescent-type gear pump, includes a metering pump for delivering fuel oil to a low pressure type atomizing nozzle.